In general, high-purity polycrystalline silicon is used as an important raw material for a semiconductor device, a solar cell, a chemical processing unit, an industrial system, or a small-sized and other highly integrated precision devices, which is respectively composed of a material with high purity or semiconducting properties.
The polycrystalline silicon is prepared using a silicon deposition method, wherein silicon atoms deposit continuously on the surface of silicon by thermal decomposition and/or hydrogen reduction of a highly-purified silicon atom-containing reaction gas.
For bulk production of polycrystalline silicon, a bell-jar type, a tube-type or a chamber-type deposition reactor has been mainly used. According to the deposition reactor, polycrystalline silicon is prepared generally in the shape of a rod with a circular or oval cross-section whose diameter is in the range of about 50-300 mm.
In the deposition reactor, a core means is basically installed for preparation of the silicon rod. For commercial production, the core means is composed of a plurality of core units respectively made of a core material (i.e., core element), through which electricity can flow at a deposition reaction temperature. The core units constituting a core means are connected to electrode units, respectively, to complete an electrical heating means in the reactor shell. Then, silicon deposits continuously on the surface of the electrically heated core means by a deposition reaction of a reaction gas comprising a silicon-containing component. As described above, the silicon deposition output is formed and enlarged in a thickness direction, that is, in an outward, radial direction of the concentric cross-section of the deposition output, and thus a rod-shaped polycrystalline silicon product can be obtained finally.
To obtain a high-purity product with minimized impurity contamination, the core units represented by a core means may be made of or fabricated with a non-contaminating core element. An ideal material for the core element is high-purity silicon that is formed like a rod, a wire or a filament, a hollow duct or a tube, a strip or ribbon, or a sheet, etc.
The polycrystalline silicon rods obtained finally by forming the deposition output around the core means are (i) divided or pulverized into the shape of chunks, nuggets, chips or particles, (ii) grouped according to size, (iii) subject to an additional step of cleaning, if required, to remove impurity components formed on the surface of silicon fragments during the pulverizing step, (iv) melted in a crucible which is heated above the melting point of silicon, and then (v) formed into an ingot, block, sheet, ribbon or film, etc., according to a use thereof.
An electrical heating means constructed within the deposition reactor shell consists of a core means which is electrically heated and an electrode means electrically connecting the core means to an electric power supply source located outside of the shell and/or electrically connecting the core units with each other. This electrical heating means serves to provide (i) an electrical heating required for maintaining a deposition reaction temperature, (ii) a starting substrate for silicon deposition, and (iii) a mechanical structure for stably supporting the silicon rod that grows in diameter and weight as the deposition continues.
Each of the core units constituting the core means should be made of or fabricated with such a core element material that satisfies the function and role of the core means. To achieve this purpose, (i) a high-purity silicon is melted alone or with a dopant component, (ii) the silicon melt is subject to crystal growing or casting, and (iii) the core element is prepared through a forming process and/or a machining process, thereby shaping its cross-section into a circle, an oval, a concentric circle or polygon, a triangle, a tetragon or a hexagon, etc.; its diameter or diagonal length may be in the range of about 3-30 mm or 5-100 mm, respectively, with its length being about 0.5-6 m.
There are several ways in preparing the core element. Each piece of the core element may be prepared in a sequential manner. Or, a plurality of core elements with a uniform size and shape may be prepared simultaneously by simply cutting a large-sized single crystal ingot. Further, a long silicon core element may be prepared by melt connection of a plurality of short pieces of core element under a clean atmosphere.
According to the description in the reference document of W. C. O'Mara, R. B. Herring and L. P. Hunt, “Handbook of Semiconductor Silicon Technology”, pp 46-48, Noyes, Publication, 1990, preparing a core element made of the high-purity silicon material, such as a core rod, a slim rod or a starter filament having a small diameter, entails a great deal of economical and technological burden in a process of preparing polycrystalline silicon rod using the deposition reactor. When the core element is made of a high-purity silicon, whose resistivity is extremely high at room temperature and drastically decreases with temperature increase, the core means begins to be electrically heated due to the occurrence of an apparent current through each core unit connected and fixed to a pair of electrode units only after the core units constituting the core means are preheated to a certain temperature or above by an additional heating means for lowering sufficiently the value of silicon resistivity. As disclosed in U.S. Pat. Nos. 4,179,530 (1979) and 5,895,594 (1999), preheating the core means for preparing a polycrystalline silicon rod requires a separate, additional preheating means and a complicated procedure.
Meanwhile, U.S. Pat. Nos. 3,941,900 (1976) and 4,215,154 (1990) disclose a technical solution to apply a direct electrical resistive heating to a core means starting from room temperature using a properly constructed electric power supply system, instead of preheating the high-purity silicon core element with a separate, additional preheating means. However, this method also has drawbacks that such an electric power supply circuit and system is highly sophisticated and costly, and requires very complicated and precise operation and control.
Unlike those methods by which the core means is preheated by a separate preheating means or is heated directly at room temperature by resistive heating using a sophisticated power supply system, incorporating a high concentration of n- or p-type dopant artificially in the silicon core element to greatly lower the resistivity enables to electrically heat up the core means directly at room temperature with high-voltage electricity. After being heated up to a predetermined temperature range, the core means can be easily heated as required with low-voltage and high-current electricity. This method has a drawback that it requires a complicated electric power supply means and a precise operation over a wide range of voltage and current.
On the other hand, if the core element is made of a non-silicon resistive material such as a metal or a carbon-based material with a resistivity value much lower than that of silicon, a silicon deposition output formed on an individual core unit can be contaminated by the impurity components generated and diffused from the core element made of a non-silicon material. However, there is an advantage that, by supplying a low-voltage electricity, the core means can be easily heated up by a resistive heating from room temperature over a deposition reaction temperature without a separate, additional preheating step. According to U.S. Pat. Nos. 5,277,934 (1994) and 5,284,640 (1994), tungsten or tantalum can be used as the core element instead of silicon. Meanwhile, U.S. Pat. No. 5,237,454 (1994) illustrates a core element made of molybdenum, tungsten or zirconium instead of high-purity silicon material.
The non-silicon core means made of a resistive material as described above can be prepared conveniently and cost-effectively. However, the deposition output obtained by silicon deposition cannot avoid being contaminated by the impurity components contained in the non-silicon core element for each of the core units constituting the core means. Thus it is difficult to apply the above method of using a non-silicon core means to a commercial production of a high-purity polycrystalline silicon rod because the purity requirement on the semiconductor-grade quality has recently become further stringent. Such a fundamental problem has also been confirmed in the prior art, as described in the above reference document (1990) of O'Mara et al. In the event a wire-type non-silicon, metallic core unit is used for the core means instead of the silicon-based core means, there is an advantage that a silicon rod product can be obtained rather conveniently. However, this method also has several disadvantages: first, when the silicon rod is finally formed as required, the deposition output and the core means included in the silicon rod should be separated with each other for the deposition output to be collected as silicon product; secondly, the deposition output formed through the silicon deposition process at a high-temperature should probably be contaminated by the impurity components out of the metallic core element.
To prepare high-purity polycrystalline silicon at a reasonable cost based on the bell-jar deposition process without any difficulties in the preheating of the silicon core means, it is worthwhile to apply a non-silicon, resistive material for the core element by solving the problems due to the replacement of the core material; the problems include a possibly difficult step for separating the core means out of the silicon rod output for collecting the silicon deposition output as product as well as a probable product contamination by the metallic impurity components out of the non-silicon core material. However, despite of the importance of the preheating of the core means, a simple, cost-effective solution has not been available to overcome those problems arising in applying the non-silicon core means.
As described above, to develop an improved method and means in preheating the core means in the bell-jar type reactor is an important technical issue for commercial bulk production of polycrystalline silicon in the form of a rod. The technical solutions required for the improvement should reduce investment costs for an electric power supply and control system and a process for preparing and machining the core means, allow an easy operation and control of the deposition reactor, enhance the reactor productivity, and ultimately lower the manufacturing cost.